The present invention relates to an electrical machine, in particular a generator for a motor vehicle.
A known electrical machine is disclosed in the European patent document EP 0 841 735 A1 and used as a water-cooled alternating or rotary current generator in motor vehicles. Its cooling casing is integrated in the cooling water circuit of the combustion engine. In the cooling water circuit also a motor cooler and air-water heat exchanger for a heating device, a circulating pump as well as regulating valve for a throughflow control of the cooling water are arranged. The unavoidable stray power of the generator can be efficiently withdrawn through the water-through flowed cooling casing. Moreover, there is the advantage that this stray power, differently from air-cooled generator, is not lost, but instead is taken by the cooling water which flows through the heat exchanger and is used in the heat exchanger for improving its feed power. This is especially advantageous for diesel motors with direct injection of multi-purpose motor vehicles, having high efficiency, since in this motor type the heating of the cooling water is moderate and allows only a little efficient inner space heating.
Accordingly, it is an object of the present invention to provide an electrical machine of the above mentioned general type, which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent herein after, one feature of present invention resides, briefly stated in an electrical machine which has a stator provided with a stator winding; a cooling casing which surrounds said stator and through which a heat transmitting medium can pass; means forming a circuit in which the heat transmitting medium circulates; at least one heat exchanger for heating purposes arranged in said circuit, said stator being provided with a selectively activatable medium for increasing a stator stray power.
The inventive electrical machine has the advantage that when a desired inner space temperature is not reached through the heating heat exchanger the stator stray power of the machine can be purposely increased when and then supplied through the heat transmitting means of the heating heat exchanger and there is used to a greater part for the inner space heating.
In accordance with a preferable embodiment of the invention, the stator stray power can be easily manipulated in that, a multi-phase or a multi-strand stator winding has at least two parallel branches per phase or strand, and switching means are provided which in each phase or in each strand selectively switch at least one branch parallel or separate from the other branch.
Thereby, for example for a three-phase stator winding with two parallel branches per phase, by switching off of one branch per phase the stator stray power in idle running of the combustion engine is increased from approximately 600 W to approximately 1100 W, 5% of current losses of the generator taken into consideration in this operational condition. This stray power which increases with increasing rotary speed of the combustion engine and thereby increasing rotary speed of the generator, provides an additional heating power for heating the interior space between 500 W and 750 W which is supplied to the heat transmitting means. These numbers are applicable for example for a rotary current generator with a stator winding switched in a triangle with an applied nominal current of 150 A.
The desired additional heating power can be realized with relatively low additional expenses, when in accordance with a preferable embodiment of the present invention for the use in a motor vehicle, the stator winding is connected to a full-wave rectifier bridge. The first ends of the switchable branches connected to the rectifier bridge are separated from the non-switchable branches in the phases or strands and are connected through each rectifier formed for example as a diode to a common tap point, and the switching means include a controllable switch which on the one hand is connected to the tap point and on the other hand is connected to one of the both bridge outlet terminals of the rectifier bridge. The additional expenses amount then to three diodes, a power-MOSFET as a switch, and a simple control electronic circuit for the MOSFET.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.